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Experimental evidence for a pure collaboration effect


What makes us willing to sacrifice our own self-interest for another person? Humans can forgo short-term individual gain to achieve long-term benefits1,2,3,4—but long-run self-interest cannot fully explain unselfish behaviour5. Collaboration in our evolutionary past may have played a role in shaping an innate human sense of distributive justice6, influencing who we consider deserving of our aid or generosity. Previous research has not been able to isolate this response to collaboration as an independent effect, distinct from other motivations to share7,8. Here we present evidence of a pure collaboration effect, distinct from motivations of future reciprocity, in-group favouritism or concern for accountability. We demonstrate this effect among adult subjects in an economic setting, showing that the effect constitutes a psychological phenomenon with relevance for real-world social and political behaviour. This collaboration effect is substantial: it motivates sharing among people otherwise inclined to share nothing and increases the proportion of participants willing to give up half of their allotted money. We find evidence supporting our hypothesis that the collaboration effect operates by creating a sense of debt owed to one’s collaborator.

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Fig. 1: Collaboration increases the amount of money given to one’s partner.
Fig. 2: The effect of collaboration is independent of the partner’s relative effort.
Fig. 3: Collaboration increases the probability of reporting indebtedness as one’s primary motivation.
Fig. 4: In-person collaboration increases the amount of money given to one’s partner.

Code availability

The code reproducing the analysis is publicly available in Northwestern University’s Arch Research and Data Repository at:

Data availability

The data sets generated during and analysed for the current study are publicly available in Northwestern University’s Arch Research and Data Repository at: These data sets include data for all figures (Figs. 14).


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Funding for experiment 4 was provided through Northwestern University’s Institute for Policy Research. The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. We thank G. Huber, J. Druckman, D. Botti, A. Fang and P. Tucker for important feedback.

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M.C.M. conducted experiment 4. M.C.M. and A.S.G. otherwise contributed equally.

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Correspondence to Mary C. McGrath.

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The authors declare no competing interests.

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Supplementary Information

Supplementary Methods and Supplementary Tables 1 and 2.

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McGrath, M.C., Gerber, A.S. Experimental evidence for a pure collaboration effect. Nat Hum Behav 3, 354–360 (2019).

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